In brief: Circadian rhythms-internal "biological clocks"-govern our physiology and behavior, influence the diagnosis and course of disease, and seem to influence athleticism. This overview describes the nature, functions, and implications of circadian rhythms, with a focus on rhythms and athleticism. It also offers a self-questionnaire to determine an individual's rhythm, which may be "lark," "owl," or intermediate. Based on the literature, the conclusions are that because of circadian rhythms, sports performance improves over the course of the day, and that it behooves athletes to know their own circadian rhythms.